What eventually happens to non-market “command” economies?

From the historical references, we can say that the two leading command economies of the world,  China and the Soviet Union both made the transition to the mixed economy.

Explanation:

A command economy refers to the economic system where all significant facets of the economy and economic production are controlled by the Government. In the command economy, it is the government that makes the decision of what to produce, how to produce, and how to distribute the manufactured products and services within the economy. It establishes a very dominant government which restricts the rights of its citizen to seek economic goals. It inevitably creates an environment in which governments will expand their influence over certain aspects of human life. Most command economies, including the Soviet Union, started making the transition to a mixed economy from the 1980s onward. This entailed a privatization process and price deregulation.

i would say their habit of breaking down.